Among the aims of the Cascadia Initiative is a comprehensive study of earthquake seismicity and risk along the coast of the northwestern U.S. ? Washington, Oregon and northern California ? where there is considerable risk of great earthquakes and tsunamis. A major goal of this research program is to provide guidance for long-term planning and hazard mitigation base on a better understanding of earthquake seismicity in the region. A major component of this program is the deployment of an array of Ocean Bottom Seismographs (OBSs). Many of these insruments will be located in shallow waters near the coast, where they will be affected by sources of noise such as waves and currents that are as yet poorly understood. The goal of this study is to obtain a good understanding of the dependence of seismic noise on OBSs location and depth to aid planning future OBS deployments and to improve methods of mitigating noise sources. The chief broader impact of this project lies in its essential contribution to planning shallow-water OBS deployments in the Cascadia Initiative and elsewhere.
Sixty-two broadband ocean bottom seismometers (OBSs) were recovered in July, 2012 following the first year deployment of the Cascadia Array. The OBSs were deployed in a diverse range of environments from water as shallow as 54 m on the continental shelf to deep water (4058 m) offshore of Washington and Oregon. Ocean bottom seismologists had a good understanding of the capabilities of standard ocean bottom seismometers in deep water, but until now there had been no long term deployments of broadband OBSs on the continental shelf. The most important question addressed was whether useful seismic data could be obtained from sensors deployed in shallow water or if the combined effects of strong currents and seafloor deformation under the loading of ocean waves would raise noise levels such that no useful seismic data could be obtained. Good observations in shallow water are critical because subduction zone earthquakes represent the greatest seismic hazard in the US and worldwide and these events are usually located beneath the shallow continental shelf. Seismic studies of subduction zones are important to understanding the level of seismic risk and to provide guidance in planning the mitigation of risk from tsunamis and earthquakes. The two main results from this study are 1) the demonstration that the large trawl shields used for the LDEO ocean bottom seismometers reduced horizontal component noise levels by more than a factor of 100 (20dB), and that 2) pressure gauges can be used to predict and remove the noise from the deformation under ocean wave loading, improving signal to noise for long period seismic phases by up to a factor of ten thousand (40dB). These results show that with proper shielding, good quality seismometer observations can be obtained from shallow water, opening up this critical part of the seafloor to observations for the first time. The data also show that without the use of pressure gauges, and without the shielding, the shallow water, long period (>10s) OBS data would be almost useless for seismology.
